I debug the logback codes and found the afterLastSlash function may caused NullPointException . Because of the function as follows.

public static String afterLastSlash(String sregex) {
          int i = sregex.lastIndexOf('/');
          if (i == -1) { 
          return sregex;
         } else {
         return sregex.substring(i + 1);
      }
}

if the "sregex" is a null String . The function will throw the exception and the program logic will be interrupted .

I will show your this case how to happed . 

I create the a spring-boot project use the logback as log framework . The logback configuration is 

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="false">
	<property name="LOG_HOME" value="log" />
	<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
		<encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
			<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{50} - %msg%n</pattern>
		</encoder>
	</appender>
	<appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
		<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
			<FileNamePattern>${LOG_HOME}/connectivi.log.%d{yyyy-MM-dd}.log</FileNamePattern>
			<MaxHistory>6</MaxHistory>
		</rollingPolicy>
		<encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
			<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{50} - %msg%n</pattern>
			<immediateFlush>true</immediateFlush>
		</encoder>
		<triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
			<MaxFileSize>10MB</MaxFileSize>
		</triggeringPolicy>
		<filter class="ch.qos.logback.classic.filter.ThresholdFilter">
			<level>TRACE</level>
		</filter>
	</appender>
	<logger name="com.mindsphere.china.poc.connectivity" level="DEBUG" additivity="false">
		<appender-ref ref="STDOUT" />
		<appender-ref ref="FILE" />
	</logger>
	<root level="ERROR">
		<appender-ref ref="STDOUT" />
		<appender-ref ref="FILE" />
	</root>
</configuration>

The program write the log into the file until the file size is small than 10MB ，When the log file size is more than 10MB . The logback continue write log will trigger the SizeBasedTriggeringPolicy . The TimeBasedRollingPolicy class will call the rollover() function . 

public void rollover() throws RolloverFailure {

// when rollover is called the elapsed period's file has
// been already closed. This is a working assumption of this method.

String elapsedPeriodsFileName = timeBasedFileNamingAndTriggeringPolicy.getElapsedPeriodsFileName();

String elapsedPeriodStem = FileFilterUtil.afterLastSlash(elapsedPeriodsFileName);

but the function timeBasedFileNamingAndTriggeringPolicy.getElapsedPeriodsFileName() return null . So the program could be broken and lose the logs.

BTW

I have choose another way to configuration the logback.xml and get the right result .But I think this is still a issues .So if you still want to have more details can connect will me by emails jbjiangbiao@icloud.com